In a hierarchical mobile telecommunications network the backhaul portion of the network comprises the intermediate links between the core network of the network and the small subnetworks at the edge of the entire hierarchical network. For example, while user equipments communicating with a base station constitute a local subnetwork, the connection between the base station and the rest of the world begins with a backhaul link to the core of the telecommunication supplier's network. Optical networks such as GPON, which provides multiple access methods, are suitable mobile backhaul solutions referred to as GPON Radio Access Networks (GPON RAN).
In a GPON RAN the Cellular Backhaul Unit (CBU) collects and distributes all kinds of traffic from the base station. Typical interface types include Ethernet (for data service) and E1/T1 (TDM for voice service). The optical line terminal (OLT) and the optical distribution Network (ODN) comprised in the GPON aggregate traffic from the Optical Network Units (ONU) (i.e. the CBUs) to the core network of the telecommunication supplier.
One of the important functions in the GPON RAN is the distribution of the real-time clock information from the core network to the base station over the GPON backhaul portion.
A prior art solution for distribution of Time-of-Day (ToD) in a GPON system is a method wherein a transaction is defined in the transmission convergence layer and a Management Entity is introduced in the management layer. The method can provide less than 1 μs phase accuracy which is adequate for mobile backhaul applications. The mentioned method and other prior art methods utilizes the real-time clock source of the OLT to adjust the real-time clock of the ONU. Hence, the prior art methods only work in the downstream direction i.e. when the time is distributed from the OLT to the ONU.
In current cellular communications network such as the CDMA based cellular network, the OLT and the base station could be geographically separated. For example, a Metropolitan Area Network (MAN) could be connected between the OLT and the base station which make the acquiring of a real-time clock source with OLT either difficult or expensive e.g. if installation of a new GPS/BITS (Global Positioning System/Building Integrated Timing Supply) is needed.